This application is based on Japanese Patent Application No. 2001-174405 filed on Jun. 8, 2001 the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a magnetic rotation angle sensor for detecting a rotation angle of a rotor relative to a stator.
2. Description of Related Art
A magnetic sensor can be used for detecting a magnetic flux modulated in accordance with a detection object, such as a rotation angle. However, the magnetic sensor detects not only the modulated magnetic flux but also an external magnetic flux. As a result, the magnetic sensor outputs a detection signal with noise caused by the external magnetic flux.
For example, FIGS. 6 and 7 show an arrangement of the magnetic sensor for detecting rotation angle. The magnetic rotation angle sensor has a stator and a rotor. One of the stator and the rotor has a magnetic flux modulator that provides a magnetic flux and modulate it in accordance with a relative rotation angle between the rotor and the stator so that the modulated magnetic flux passes through the other one of the stator and the rotor. The other one of the rotor and the stator has a magnetic sensor element for detecting the modulated magnetic flux, and outputs a signal indicative of the rotation angle. For example, the magnetic flux modulator may be provided by cores that vary a direction of the magnetic flux or an amount of the magnetic flux in accordance with a relative rotational position of the rotor and the stator.
In the illustrated case, the magnetic rotation angle sensor has a first portion that has a yoke 13. The yoke 13 is formed in a substantially cylindrical shape to define an inner cavity. The yoke 13 is made of magnetic material and defines two gaps 11 for receiving magnets 12 respectively. The gaps 11 are located in a diametric relationship on the yoke 13 and separate the yoke 13 into two portions. The magnets 12 are magnetized in tangential directions to a circle defined by the yoke 13. The magnetized axes D1 and D2 are parallel with each other, and directed in the same direction. A second portion that has a core 16 and a magnetic sensor element 15 is disposed inside the first portion. The core 16 is made of magnetic material. The core 16 defines a gap 14 that separates the core 16 into two members at a diametric line. Two magnetic sensor elements 15 are disposed in the gap 14. Each of the magnetic sensor elements 15 is a molded IC including the Hall effect element. The magnets 12 provide a magnetic flux passing through the yoke 13, the core 16 and the magnetic sensor element 15. A relative rotation of the first portion and the second portion changes a relative positional relationship between the yoke 13 and the core 16. The magnetic flux is modulated in accordance with the relative positional relationship between the yoke 13 and the core 16. The magnetic sensor element 15 outputs a signal in accordance with the modulated magnetic flux that is indicative of relative rotation angle between the first and second portions. For example, the yoke 13 may rotate as the rotor and the core 16 may be fixed as the stator.
According to the arrangement described above, if the sensor is disposed in an external magnetic field, the sensor is influenced by the external magnetic field. For example, if an external magnetic flux may pass the core 16 in a diagonal direction through a path indicated by a broken line P4 as shown in FIG. 6, an output signal of the magnetic sensor element 15 indicates an incorrect angle.
In a case in which the external magnetic field is applied in a direction perpendicular to a rotation axis of the rotor, the magnetic flux may pass the yoke 13 and the core 16 in a diametric direction through a path indicated by a broken line P5 as shown in FIG. 7. In a case of above, the magnetic sensor element 15 also outputs an incorrect signal.
It is an object of the present invention to provide a magnetic sensor that is capable of reducing an influence of an external magnetic field.
It is another object of the present invention to provide a magnetic sensor that is capable of detecting a rotation angle accurately.
It is a still another object of the present invention to provide a magnetic sensor that is capable of reducing an influence of an external magnetic field while preventing a short-circuiting leakage of the magnetic flux that is provided by a magnet.
According to a first aspect of the present invention, a magnetic rotation angle sensor for detecting a rotation angle of a rotor relative to a stator comprises a magnet, a yoke, a core, and a magnetic sensor element that detects the magnetic flux modulated by the core. A first means is provided for providing an external magnetic path for an external magnetic flux. The external magnetic path avoids the magnetic sensor element, and has a magnetic resistance lower than that of a magnetic path passing through the magnetic sensor element. Therefore, the external magnetic path guides the external magnetic flux rather than the magnetic path passing through the magnetic sensor element. As a result, the influence of the external magnetic field is reduced. A second means is provided for reducing short-circuiting leakage of the magnetic flux supplied by the magnet. Therefore, the yoke and the core receives sufficient amount of the magnetic flux from the magnet and the magnetic sensor element outputs accurate detection signal.
According to another aspect of the present invention, an additional external magnetic guide provides the external magnetic path. The external magnetic guide provides an axially extending external magnetic path.
According to a still another aspect of the present invention, the magnet is magnetized in an oblique direction so as to provide a shorter gap between the yokes and a longer distance between the poles. This arrangement provides the external magnetic path in the yoke, and provides a sufficient distance between the poles to reduce short-circuiting leakage of the magnetic flux supplied by the magnet.